The present invention relates to belt supporting and tracking apparatus and more particularly to apparatus for controlling the lateral movement of the belt from its predetermined path.
In an electrostatographic reproducing apparatus commonly in use today, a photoconductive insulating member is typically charged to uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or back ground areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the usual document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developing powder referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas in the photoconductive insulating area to form a powder image on the photoconductive area. This image may subsequently be transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure.
Many commercial applications of the above process employ the use of the photoconductive insulating member in the form of a belt which is supported about a predetermined path past the plurality of processing stations to ultimately form a reproduced image on copy paper. The location of the latent image recorded on the photoconductive belt must be precisely defined in order to have the various processing stations acting thereon optimize copy quality. To this end it is critical that the lateral alignment of the photoconductive belt be controlled within prescribed tolerances. Only in this manner will a photoconductive belt move through a predetermined path so that the processing stations disposed thereabout will be located precisely relative to the latent image recorded thereon.
When considering control of the lateral movement of the belt, it is well known that if the belt were perfectly constructed and entrained about perfect cylindrical rollers mounted and secured in an exactly parallel relationship with one another, there would be no lateral movement of the belt. In actual practice, however, this is not feasible. Due to the imperfections in the system geometry, the belt velocity vector is not normal to the roller axis of rotation, and the belt will move laterally relating to the roller until reaching a kinematically stable position. Existing methods of controlling belt lateral movement comprise servo systems, crowned rollers and flanged rollers. In any control system, it is necessary to prevent high local stresses which may result in damage to the highly sensitive photoconductive belt. Active systems, such as servo systems employ steering rollers which apply less stress on the belt. However, active systems of this type are generally complex and costly. Passive systems, such as flanged rollers, are less expensive, but generally produce high stresses. Various types of flanged rollers systems have hereinbefore been developed to improve the support and tracking of photoconductive belts. For example, the drive roller may have a pair of flanges secured to opposed ends hereof. If the photoconductive belt moves laterally, and engages one of the flanges, it must be capable of either sliding laterally with respect to the roller system, or locally deforming either itself or the roller system to maintain its position. The edge force required to shift the belt laterally or locally deform itself on the roller system usually greatly exceeds the maximum tolerable edge force. Thus, the belt would start to buckle resulting in failure of the system. Alternatively, the flanges may be mounted on one of the idler rollers rather than the drive roller. Lateral motion is controlled by bending the belt to change the approach angle to the drive roller. A system of this type may develop low edge forces when compared to having the flanges mounted on the drive roller. However, the primary risk associated with this system is that performance depends significantly on the belt bending in its plane. Although the forces in this type of a system are often reduced, they still appear to be unacceptable in that they generally exceed the belt buckling force. Thus, the side edge of the photoconductive belt eventually buckles reducing the lift thereof.